Parsing: Replace `Biased` with a priority system.

[paulstansifer]

Nested Alts and Biaseds are (a) hard to understand, and (b) impossible to add to, making ::=also limited in what it can do.

Instead:

• Make ::=also assume that there's a top-level Alt and add to it, rather than adding another layer of nesting.
• Get rid of Biased, and instead have some sort of Priority wrapper that sets a rule's priority.

That way, rules can be usefully prioritized when extending grammars.

This shouldn't be too hard. It will require adjusting LocalParse, but that it's just a matter of changing JustifiedByAtomPlanB and thinking through how prioritized and non-prioritized rules relate to each other. (Maybe they have priority 0, and you can set priorities both positive and negative?) Might also need to think about when a partial parse "loses" its priority.

[paulstansifer]

I don't like that this adds numbers to the core language. But I think it's the only reasonable approach.

I guess there's already numeric types, and one through ten, and anything_to_string even assumes base-10. But there's no way to relegate this to core_extra_forms.rs.